High molar extinction coefficient heteroleptic ruthenium complexes for thin film dye-sensitized solar cells

Article abstract of DOI:10.1021/ja058540p

Two novel heteroleptic sensitizers, Ru((4,4-dicarboxylic acid-2,2′-bipyridine)(4,4′-bis(p-hexyloxystyryl)-2,2-bipyridine) (NCS)₂, and Ru((4,4-dicarboxylic acid-2,2′-bipyridine)(4, 4′-bis(p-methoxystyryl)-2,2′-bipyridine) (NCS)₂, code

Full text of DOI:10.1021/ja058540p

Published on Web 03/07/2006
High Molar Extinction Coefficient Heteroleptic Ruthenium
Complexes for Thin Film Dye-Sensitized Solar Cells
Daibin Kuang, Seigo Ito, Bernard Wenger, Cedric Klein, Jacques-E Moser,
Robin Humphry-Baker, Shaik M. Zakeeruddin,* and Michael Gra¨tzel*
Contribution from the Laboratory for Photonics and Interfaces, Institute of Chemical Sciences
and Engineering, Ecole Polytechnique Fe´de´rale de Lausanne, 1015 Lausanne, Switzerland
Received December 16, 2005; E-mail: shaik.zakeer@epfl.ch; michael.graetzel@epfl.ch
Abstract: Two novel heteroleptic sensitizers, Ru((4,4-dicarboxylic acid-2,2′-bipyridine)(4,4′-bis(p-hexyloxy-
styryl)-2,2-bipyridine)(NCS)2 and Ru((4,4-dicarboxylic acid-2,2′-bipyridine)(4,4′-bis(p-methoxystyryl)-2,2′-
bipyridine) (NCS)2, coded as K-19 and K-73, respectively, have been synthesized and characterized by 1H
NMR, FTIR, UV-vis absorption, and emission spectroscopy and excited-state lifetime and spectroelec-
trochemical measurements. The introduction of the alkoxystyryl group extends the conjugation of the
bipyridine donor ligand increasing markedly their molar extinction coefficient and solar light harvesting
capacity. The dynamics of photoinduced charge separation following electronic excitation of the K-19 dye
was scrutinized by time-resolved laser spectroscopy. The electron transfer from K-19 to the conduction
band of TiO2 is completed within 20 fs while charge recombination has a half-life time of 800 µs. The high
extinction coefficients of these sensitizers enable realization of a new generation of a thin film dye sensitized
solar cell (DSC) yielding high conversion efficiency at full sunlight even with viscous electrolytes based on
ionic liquids or nonvolatile solvents. An unprecedented yield of over 9% was obtained under standard
reporting conditions (simulated global air mass 1.5 sunlight at 1000 W/m2 intensity) when the K-73 sensitizer
was combined with a nonvolatile “robust” electrolyte. The K-19 dye gave a conversion yield of 7.1% when
used in conjunction with the binary ionic liquid electrolyte. These devices exhibit excellent stability under
light soaking at 60 °C. The effect of the mesoscopic TiO2 film thickness on photovoltaic performance has
been analyzed by electrochemical impedance spectroscopy (EIS).
(SCN)3L (L ) 4,4′,4′′-tricarboxy 2,2′:6′,2′′-terpyridine)6 showed
Introduction
the best performance as sensitizers so far due to their advanta-
geous spectral properties and high stability. While more than
11% conversion efficiency has been reached with the N3 dye,
a film thickness of over 15 microns and a volatile redox
electrolyte were required to achieve this performance. The long-
term containment at elevated temperatures of the volatile solvent
mixture employed still remains a major challenge.7
This dilemma is currently being addressed by the development
of novel sensitizers with an increased optical cross section
allowing thinner TiO2 films and nonvolatile electrolytes to be
employed. A successful approach has been to replace one of
the 2.2′-bipyridyl-4,4′-dicarboxylate groups in the N3 dye by a
styryl-subsituted bipyridine. This not only increases the extinc-
tion coefficient of the sensitizer by extending the π-conjugation
of the ligand but also augments its hydrophobicity, preventing
dye desorption by water and stabilizing device performance
under long-term light soaking and thermal stress.
Following its discovery in 1991,1 research on the dye-
sensitized solar cell (DSC) has progressed remarkably, rendering
it a credible chemical alternative to solid-state silicon based
devices.2-5 The high efficiency of the DSC arises from the
collective effect of numerous well-tuned physical-chemical
properties, the key issue being the panchromatic sensitization
of large band-gap mesoscopic semiconductor electrodes. Ru-
thenium polypyridyl complexes such as the cis-Ru(SCN)2L2 (L
) 2.2′-bipyridyl-4,4′-dicarboxylate (N3) or the black dye Ru-
(1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737.
(2) (a) Benkstein, K. D.; Kopidakis, N.; van de Lagematt, J. Frank, A. J. J.
Phys. Chem. B 2003, 7759. (b) Benko¨, G.; Skårman, B.; Wallenberg, R.;
Hagfeldt, A.; Sundstro¨m, V.; Yartsev, A. P. J. Phys. Chem. B 2003, 107,
1370. (c) Nakade, S.; Saito, Y.; Kubo, W.; Kitamura, T.; Wada, Y.;
Yanagida, S. J. Phys. Chem. B 2003, 107, 8607.
(3) (a) Gra¨tzel, M. Chem. Lett. 2005, 34, 8-13. (b) Yanagida, M.; Yamaguchi,
T.; Kurashige, M.; Fujihashi, G.; Hara, K.; Katoh, R.; Sugihara, H.;
Arakawa, H. Inorg. Chim. Acta 2003, 351, 283. (c) Heimer, T. A.; Heilweil,
E. J.; Bignozzi, C. A.; Meyer, G. J. J. Phys. Chem. A 2000, 104, 4256.
(4) Adachi, M.; Murata, Y.; Takao, J.; Jiu, J.; Sakamoto, M.; Wang, F. J. Am.
Chem. Soc. 2004, 126, 14943. (b) Ushiroda, S.; Ruzycki, N.; Lu, Y.; Spitler,
M, T.; Parkinson, B. A. J. Am. Chem. Soc. 2005, 127, 5158.
(5) For example, see: (a) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.;
Humphry-Baker, R.; Gra¨tzel, M. J. Am. Chem. Soc. 2004, 126, 7164. (b)
Sapp, S. A.; Elliott, M.; Contado, C.; Caramori, S.; Bignozzi, C. A. J. Am.
Chem. Soc. 2002, 124, 11215. (c) Nusbaumer, H.; Zakeeruddin, S. M.;
Moser, J.-E.; Gra¨tzel, M. Chem.sEur. J. 2003, 9, 3756. (d) Balzani, V.;
Campagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi, M. Acc. Chem.
Res. 1998, 31, 26. (e) Hara, K.; Kurashige, M.; Dan-oh, Y.; Kasada, C.;
Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. New J. Chem. 2003, 783.
(f) Horiuchi, T.; Miura, H.; Uchida, S. Chem. Commun. 2003, 3036.
Following our recent communication on the K-19 dye Ru-
(4,4′-dicarboxylic acid-2,2′-bipyridine)(4,4′-bis(p-hexyloxystyryl)-
(6) (a) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.;
Humphry-Baker, R.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia,
L.; Deacon, G. B.; Bignozzi C. A.; Gra¨tzel, M. J. Am. Chem. Soc. 2001,
123, 1613. (b) Nazeeruddin, M. K.; Kay, A.; RodicioI,; Humphry-Baker,
R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc.
1993, 115, 6382.
(7) Gra¨tzel, M. Inorg. Chem., 2005, 44 (20), 6841.
9
4146
J. AM. CHEM. SOC. 2006, 128, 4146-4154
10.1021/ja058540p CCC: $33.50 © 2006 American Chemical Society

High Molar Extinction Coefficient Heteroleptic Ru Complexes
A R T I C L E S
2,2′-bipyridine)(NCS)2,8 an in-depth analysis of this promising
new generation of sensitizers is performed in the present study.
This includes a comparison of the K-19 with its less hydrophobic
analogue Ru(4,4-dicarboxylic acid-2,2′-bipyridine)(4,4′-bis(p-
methoxystyryl)-2,2′-bipyridine)(NCS)2 coded as K-73, where
the hexyl chains are replaced by methyl groups.
18H), 4.05 (s, 3H), 4.0 (s, 3H), 3.2 (t, 8H), 2.8 to 1 (m, 28H). Anal.
Calcd for RuC58H67N7O6S2: C, 62.01; H, 6.01; N, 8.73%. Found: C,
62.37; H, 6.35; N, 8.6%.
Instrumentation. 1H NMR spectra were measured on a Bruker 200
MHz spectrometer. The 1H spectra were referenced to tetramethylsilane.
Elemental analysis was carried out at the Institute of Chemical Science
and Engineering at the EPFL. UV-vis and photoluminescence spectra
were measured using a 1-cm path length quartz cell and a Cary 5
spectrophotometer or Spex Fluorolog 112 spectrofluorometer, respec-
tively. The emitted light was detected with a Hamamatsu R2658
photomultiplier operated in the single-photon counting mode. The
emission spectrum was photometrically corrected with a calibrated 200
W tungsten lamp as the reference source.
Experimental Section
Reagents. The high molar extinction coefficient sensitizer K-19, Ru-
(4,4-dicarboxylic acid-2,2′-bipyridine)(4,4′-bis(p-hexyloxystyryl)-2,2′-
bipyridine)(NCS)2 was synthesized as reported earlier.8 The molecular
structures of sensitizer K-19 and K-73 are shown in Figure S1. Ionic
liquids including 1-propyl-3-methylimidazolium iodide (PMII), 1-ethyl-
3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI), and
1-ethyl-3-methylimidazolium thiocyanate (EMINCS) were prepared
according to literature methods, and their purities were confirmed by
1H NMR spectra.9 The coadsorbents 3-phenylpropionic acid (PPA) and
guanidinium thiocyanate (GuNCS) were purchased from Aldrich, and
1-decylphosphonic acid (DPA) from Lancaster was used as received.
N-Methylbenzimidazole (NMBI, from Aldrich) was recrystallized from
diethyl ether before use.
Synthesis of 4,4′-Bis(2-(4-methoxyphenyl)styryl)-2,2′-bipyridine.
Solid tert-BuOK (7.5 g, 67 mmol) was added to a solution of 4,4′-
dimethyl-2,2′-bipyridine (3.0 g, 16 mmol) and 4-methoxybenzaldehyde
(5.5 g, 40 mmol) in anhydrous DMF (100 mL). The resulting mixture
was stirred 24 h at room temperature under nitrogen. The solvent was
evaporated, and methanol (200 mL) was added. The insoluble solid
was filtered on a sintered crucible and recrystallized from hot acetic
acid, filtered, and washed with methanol to obtain the desired product
as a beige solid (4 g, 58%).
Synthesis of Ru(4,4′-bis(2-(4-methoxyphenyl)styryl)-2,2′-bipy-
ridine)(p-cymene)(Cl)2. A mixture of 4,4′-bis(2-(4-methoxyphenyl)-
styryl)-2,2′-bipyridine (0.5 g, 1.2 mmol) ligand and [Ru(Cl)2(p-
cymene)]2 (0.365 g, 0.59 mmol) in ethanol (80 mL) was refluxed for
4 h under nitrogen. After evaporation of the solvent, pure Ru(4,4′-bis-
(2-(4-methoxyphenyl)styryl)-2,2′-bipyridine)(p-cymene)(Cl)2 complex
was left as an orange colored oil. 1H NMR (200 MHz, 25 °C, CDCl3)
δ 0.83 (s, 3H), 0.87 (s, 3H), 2.16 (s, 3H), 2.35 (m, 1H), 3.66 (s, 6H),
5.84 (d, J ) 5.8 Hz, 2H), 5.92 (d, J ) 5.8 Hz, 2H), 6.75 (m, 6H), 7.36
(d, J ) 5.5 Hz, 2H), 7.51 (d, J ) 8.5 Hz, 4H), 7.74 (d, J ) 16 Hz,
2H), 8.57 (s, 2H), 9.23 (d, J ) 5.5 Hz, 2H).
ATR-FTIR Measurements. The spectra for all the samples were
measured using a Digilab FTS 7000 FTIR spectrometer fitted with a
DTGS detector. All the data reported here were taken with the “Golden
Gate” 45° diamond anvil ATR accessory. Spectra were derived from
64 scans at a resolution of 2 cm-1. Prior to measuring the spectra, the
dyed films were rinsed with acetonitrile solvent to washout any weakly
adsorbed molecules and dried.
Electrochemical Impedance Measurements. Impedance spectra of
fully assembled DSCs having different film thicknesses were measured
in the dark at -0.68 V applied forward bias or under simulated AM1.5
global illumination (450 W xenon lamp equipped with appropriate
filters) at open circuit using a computer controlled potentiostat (EG&G,
M273) equipped with a frequency response analyzer (EG&G, M1025).
The spectra were scanned in a frequency range of 0.005 Hz-100 kHz
at room temperature with an alternating voltage amplitude set at 10
mV.
Oxidation of Sensitizer. Electrochemical oxidation was carried out
with a three-electrode spectroelectrochemical cell. TBAPF6 (0.1 M)
dissolved in acetonitrile was used as the electrolyte. The electrode
potential was adjusted by a PC-controlled AutoLab PSTAT10 electro-
chemical workstation (Eco Chimie). The UV-vis spectra were recorded
by polarizing the electrode at 0.8 V vs NHE until the current drops to
1
/
of the initial value.
10
Femtosecond Transient Spectrometer. A detailed description of
the setup used for time-resolved transient absorption measurements has
been given in a previous paper.10 Briefly, the system is based on a
Ti:sapphire amplified fiber laser source (Clark-MXR CPA 2001)
providing pulses at a 1 kHz repetition rate centered at 775 nm with a
duration of 120 fs and a pulse energy of 1 mJ. One part of the beam
is used to pump a noncollinear parametric amplifier (NOPA) to produce
the pump pulse centered at 520 nm with a typical duration of 30 fs
after compression. The supercontinuum probe is generated by focusing
a small part of the fundamental beam (<3 µJ) in a 2 mm sapphire
plate. The white light continuum was collimated with a parabolic mirror,
split into a probe and reference beam with a thin metallic beam splitter,
and focused on the sample with another parabolic mirror in order to
minimize chirp. The energy of the pump beam was reduced to 1 µJ
before focusing on the sample, and the relative polarization of the beams
was adjusted at the magic angle. The reference and signal beams were
analyzed in a spectrograph (Triax 320, Jobin-Yvon) and detected by a
double photodiode array with each 1024 elements (Princeton Instru-
ments).
In the two-color pump-probe scheme, parts of the fundamental beam
are used to pump two independent NOPAs that are tuned in order to
provide pulses centered at 535 nm for the pump and 890 nm for the
probe. After compression, the laser beams are focused on the sample
by fused silica lenses. Neutral density filters are used to ensure intensity-
independent dynamics. To avoid dye degradation, the sample is
constantly rotated. For detection, the signal of a photoreceiver (Nirvana
2007, New Focus) is sent to a lock-in amplifier (SR-830, Stanford
Research Instruments) tuned at the frequency of a chopper selecting
one pulse over two (500 Hz).
Ru(4,4′-bis(2-(4-methoxyphenyl)styryl)-2,2′-bipyridine)(4,4′-di-
carboxy-2,2′-bipyridine)(NCS)2. A Ru(4,4′-bis(2-(4-methoxyphenyl)-
styryl)-2,2′-bipyridine)(p-cymene)(Cl)2 and 4,4′-dicarboxy-2,2′-bi-
pyridine (0.291 g, 1.2 mmol) in DMF (60 mL) were heated to 140 °C
for 4 h under nitrogen in the dark. To the green reaction mixture was
then added NH4NCS (2 g, 26 mmol), and the heating was continued
for another 4 h. The reaction mixture was cooled to room temperature,
and after the DMF was evaporated, water (200 mL) was added and the
resulting purple solid was filtered and washed with water. The crude
compound was dissolved in basic methanol (tetrabutylammonium
hydroxide) solution and purified by passing through a Sephadex LH-
20 column with methanol as the eluent. After collecting the main band
and evaporating the solvent, the resultant solid was redissolved in water
and the pH was lowered to 4.8 by titrating with dilute nitric acid to
obtain the K-73 complex, containing one proton and one tetrabuty-
lammonium cation (TBA+), as a precipitate. The final product was
1
washed thoroughly with water and dried under a vacuum. H NMR
(δH/ppm in CD3OD+ NaOD) 9.45 (d, 1H), 9.20 (d, 1H), 8. 95 (s, 1H),
8.80 (s, 1H), 8.30 (s, 1H), 8.15 (s, 1H), 8.00 (d, 1H), 7.90 to 6.80 (m,
(8) (a) Wang, P.; Klein, C.; Humphry-Baker, R.; Zakeeruddin, S. M.; Gra¨tzel,
M. J. Am. Chem. Soc. 2005, 127, 808. (b) Wang, P.; Klein, C.; Humphry-
Baker, R.; Zakeeruddin, S. M.; Gra¨tzel, M. Appl. Phys. Lett. 2005, 86,
123508.
(9) Pringle, J. M.; Golding, J.; Forsyth, C. M.; Deacon, G. B.; Forsyth, M.;
MacFarlane, D. R. J. Mater. Chem. 2002, 12, 3475.
(10) Pelet, S.; Gra¨tzel, M.; Moser, J. E. J. Phys. Chem. B 2003, 107, 3215.
9
J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006 4147

A R T I C L E S
Kuang et al.
The samples were made of 8 µm thick TiO2 films soaked overnight
in a 0.3 mM K-19 dye solution in a 1:1 mixture of t-BuOH/CH3CN.
After dye absorption the films were covered with the redox inactive
ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-
imide) or when indicated, with the nonvolatile electrolyte described
below. The optical densities of the films were typically 2.0 at 527 nm.
Al2O3 films were prepared as previously described,11 and dye adsorption
was accomplished following the same procedure as that for TiO2.
from the counter electrode side through a predrilled hole, and then the
hole was sealed with a Bynel sheet and a thin glass slide cover by
heating. The nonvolatile electrolyte contains 0.8 M PMII, 0.15 M iodine,
0.1 M GuNCS, and 0.5 M NMBI in 3-methoxypropionitrile. The
solvent-free ionic liquid electrolyte contains 0.2 M iodine, 0. 5 M
NMBI, and 0.1 M GuNCS in a mixture of PMII/EMINCS (13:7 vol
ratio).
Photocurrent-Voltage Measurements. The irradiation source for
the photocurrent-voltage (I-V) measurement is a 450 W xenon light
source (Osram XBO 450, USA), which simulates the solar light. The
incident light intensity was calibrated with a standard Si solar cell.
The spectral output of the lamp matched precisely the standard global
AM 1.5 solar spectrum in the region 350-750 nm (mismatch < 2%)
by the aid of a Schott K113 Tempax sunlight filter (Pra¨zisions Glas &
Optik GmbH, Germany). Various irradiance intensities (from 0.01 to
1.0 sun) can be provided with neutral wire mesh attenuators. The
current-voltage curves were obtained by measuring the photocurrent
of the cells using a Keithley model 2400 digital source meter (Keithley,
USA) under an applied external potential scan. The transient current
dynamics characterization of the cell was obtained using the same
system of I-V measurement. The measurement of incident photon-to-
current conversion efficiency (IPCE) was performed by a similar data
collecting system but under monochromatic light. IPCE was plotted as
a function of excitation wavelength. The incident light from a 300 W
xenon lamp (ILC Technology, USA) was focused through a Gemini-
180 double monochromator (Jobin Yvon Ltd., UK) onto the cell under
test.
Nanosecond Laser Transient Absorbance Measurements. Dye-
coated, 8 µm thick transparent nanocrystalline TiO2 films were excited
by nanosecond laser pulses produced by a broad-band optical parametric
oscillator (OPO GWU-355), pumped by a frequency-tripled Q-switched
Nd:YAG laser (Continuum Powerlite 7030). Excitation pulses (λ )
600 nm, 30 Hz repetition rate, pulse width at half-height of 7 ns) were
attenuated by neutral density filters to reduce pulse fluence on the
sample down to 25 µJ cm-2. The probe light from a Xe arc lamp was
passed through a monochromator, various optical elements, the sample,
and a second monochromator before being detected by a fast photo-
multiplier tube. Typically, averaging over 3000 laser shots was
necessary to get satisfactory signal/noise levels.
Preparation of Mesoscopic TiO2 Films. The mesoscopic TiO2 film
used as photoanode consisted of two layers. A transparent, high surface
area layer of 20 nm TiO2 particles was first printed on the fluorine-
doped SnO2 (FTO) conducting glass electrode obtained from LOF
(TEC-15, sheet resistance 15 Ohm/square). Subsequently a second layer
of 400 nm light scattering anatase particles was applied as reported,12
with some modifications as explained below. The FTO glass was
washed with ethanol and treated by UV-O3 for 20 min in order to
remove organics or contaminants. Thereafter it was placed in a 40 mM
TiCl4 aqueous solution at 70 °C for 30 min to enhance adhesion between
the TiO2 particles and the FTO surface. The transparent TiO2 film of
3.4 µm thickness (particle size: 20 nm) was screen-printed on the clean
FTO glass as described above. A thicker transparent TiO2 layer can be
obtained by repeating the operation. Thinner films (1 or 2 µm) were
made by a similar procedure using a diluted TiO2 paste. In the present
study, film thicknesses of 1, 2, 3.4, 6.8, 10.2, and 13.6 µm were
investigated. To enhance light harvesting in the red and near-IR spectral
region, another layer was screen-printed by using a paste of 400 nm
light scattering TiO2 particles. The thickness of the scattering layer
was kept constant at 4 µm. The layer thickness was determined by
using an Alpha-Step 200 Surface Profilometer (Tencor Instruments,
USA). The double layer composed of the transparent and light scattering
particle was gradually heated to 500 °C and sintered for 15 min. The
sintered double films were further treated with 40 mM TiCl4 aqueous
solution at 70 °C for 30 min, then washed with water and ethanol, and
stored in dry air prior to device fabrication. The porosity and pore size
of the TiO2 paste used for the transparent layer as characterized by
Gemini 2327 nitrogen adsorption apparatus (Nicromeretics Instrument
Corp., USA) were 63% and 22 nm, respectively.
Light-Soaking Stability. Hermetically sealed cells were used to
check the long-term stability under thermal stress (60 °C) and visible
light soaking. The cells were covered with a polymer film of 50 µm
thickness (Preservation Equipment Ltd, UK), which acts as a UV cutoff
filter, and were illuminated at open circuit under a Suntest CPS lamp
(ATLAS GmbH, 100 mW/cm2, 60 °C). The cells were taken out at
regular intervals to record the photocurrent-voltage curve measured
over a period of 1000 h.
Results and Discussion
1. Molecular Orbital Calculations. The Spartan commercial
software was used to calculate the molecular orbitals of the K-73
heteroleptic ruthenium sensitizers and thereby the photocon-
version efficiency. The profiles for the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) are presented in Figure S2. The LUMO is
localized on the 4,4-COOH 2,2′-bipyridine with the electron
density being highest on the oxygen atoms of the carboxylate
groups while the HOMO is shared by the Ru and NCS ligands.
2.1. Redox Potential. Cyclic voltammetry was performed to
measure the redox potential of the sensitizer K-73 and the result
are shown in Figure S3. For the K-73 dye adsorbed on the TiO2
film, the average of the peak potentials of the cathodic and
anodic wave was at 1.01 V vs. normal hydrogen electrode
(NHE). This value is about 0.6 V more positive than the
potential of the iodide/triiodide couple in the redox electrolyte
providing a large driving force for the dye regeneration and
thus net charge separation.
2.2. Electronic, Emission Spectra, and Exited State Life-
time. The two heteroleptic ruthenium complexes, K-19 and
K-73, exhibit two intense broad absorption bands in the visible
region at 410 and 545 nm due to metal-to-ligand charge-transfer
(MLCT) transitions (Figure S4). In addition there are two
features in the UV region. As observed in K-19 dye,8a the narrow
band at 310 nm arises from the π-π* transition of the 4,4′-
dicarboxy 2,2′-bipyridine ligand, and the broad band at 350 nm
Fabrication of DSC Devices. The double layer films were heated
to 520 °C and sintered for 30 min, then cooled to ∼80 °C and immersed
into the dye solution at room temperature for 16 h. The dye solution
containing 300 µM K-19 or K-73 in acetonitrile and tert-butyl alcohol
(volume ratio: 1:1) was used to sensitize the photoanode. In the
presence of coadsorbent, 300 µM PPA or 75 µM DPA was added to
the above dye solution. Dye coated double layer films were assembled
and sealed with a thin transparent hot-melt 25 µm thick Surlyn ring
(DuPont) to the counter electrodes (Pt on FTO glass, chemical
deposition of 0.05 M hexachloroplatinic acid in 2-propanol at 400 °C
for 15 min). The electrolyte was injected into the interelectrode space
(11) Nu¨esch, F.; Shklover, V.; Moser, J. E.; Gra¨tzel, M. J. Am. Chem. Soc.
1996, 118, 5420.
(12) (a) Barbe´, C. J.; Arendse, F.; Comte, P.; Jirousek, M.; Lenzmann, F.;
Shklover, V.; Gra¨tzel, M. J. Am. Ceram. Soc. 1997, 80, 3157. (b) Wang,
P.; Zakeeruddin, S. M.; Comte, P.; Charvet, R.; Humphry-Baker, R.;
Gra¨tzel, M. J. Phys. Chem. B 2003, 107, 14336.
9
4148 J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006

High Molar Extinction Coefficient Heteroleptic Ru Complexes
A R T I C L E S
is assigned to the π-π* transition of the 4,4′-bis(2-(4-meth-
oxyphenyl)styryl)-2,2′-bipyridine having an extended π system.
The light harvesting efficiency is primarily governed by the
molar extinction coefficient of the dye. For Z 907Na dye, the
molar extinction coefficient is 33% lower than those of the
extended conjugation dyes (K-19 or K-73) in solution. In terms
of the optical density of the adsorbed dyes on a 8 µm thick
transparent film, there is a 35% decrease for Z 907Na dye over
the K-73 or K-19 dyes (Figure S5). This result indicates that
there is only a small change in the amount of adsorbed dyes on
the film.
Excitations of the low energy MLCT transition of the K-73
dye in DMF solvent produces an emission centered at 830 nm
(Figure S4). The emission spectrum was further analyzed by
applying a Gaussian reconvolution method, which enabled
determination of the integral under the emission peak, devoid
of Raman and other instrumental artifacts. Knowledge on the
formal redox potential of the excited state [φ0(S+/S*)] of a dye
relative to the conduction band energy level of a TiO2
semiconductor allows a priori determination of the direction of
current flow. An approximate value of φ0(S+/S*) of a dye can
be extracted from the formal potential of the ground state [φ 0-
(S+/S)] and its excitation energy (E0-0) according to eq 1 (where
F is the Faraday constant) obtained by electrochemical and
spectroscopic measurements.
Figure 1. Base vector from the SVD of K19 observed on TiO2 following
excitation at 520 nm (0) and on Al2O3 after excitation at 500 nm (4). The
inset shows the singular values of the decomposition for the data obtained
on TiO2 (O).
peaks located at 1538 cm-1 and 1595 cm-1 arise from the
aromatic ring modes.
3. Oxidized Form of the Sensitizer. The mesoscopic TiO2
film of 6 µm thickness deposited on FTO glass was stained
with a K-19 dye solution at room temperature for 16 h and used
as a working electrode in a three electrode spectroelectrochemi-
cal cell. Acetonitrile was used as the solvent with TBA (PF6)
as the supporting electrolyte. The adsorbed monolayer of K-19
was oxidized by hole injection from the FTO support by
applying a bias potential of 0.8 V vs NHE. Figure S8 shows
the visible spectrum of the oxidized form of the K-19 sensitizer
adsorbed on the mesoscopic TiO2 film. Oxidation of K-19 results
in bleaching of the low energy band at 545 nm with a growing
feature centered at 775 nm. The weaker red shifted transition
is assigned as a ligand-to-metal charge transition (LMCT) of
the oxidized K-19 sensitizer. Previous investigations show that
oxidation from ruthenium(II) to ruthenium(III) bipyridyl com-
plexes results in a red shift in absorption and correspondingly
weaker band intensities.14 From the absorption decay the half-
life time of the oxidized K-19 sensitizer was determined as 2.6
min.
4. Ultrafast Electron Injection Kinetics. Time-resolved
absorption changes were measured after excitation at 520 nm
over a wavelength range extending from 470 to 690 nm. To
get the best resolution for the temporal and spectral behavior,
a singular value decomposition (SVD) was performed on the
data matrix of which the rows and columns correspond to the
single wavelength and time position vectors, respectively. This
purely mathematical technique decomposes the initial matrix
into two linearly independent bases and assigns a singular value
to each of the base vectors. The number of nonzero singular
values gives the rank of the matrix, i.e., in our case the number
of the transient chemical species.15 The SVD of the measured
transient data show clearly one dominating singular value
suggesting that a single chemical species is present (see inset
of Figure 1). The row vector associated with this value gives
the spectral shape of this species. The profile of this transient
φ0(S+/S*) ) φ0(S+/S) - E0-0/F
(1)
As presented in Figure S3, the formal redox potential of K-73
dye adsorbed on the surface of TiO2 film obtained by averaging
the anodic and cathodic peak potentials was 1.01 V vs NHE
(normal hydrogen electrode). E0-0 transition energy was esti-
mated to be 1.72 eV, and thus φ0(S+/S*) of K-73 was calculated
to be -0.71 V vs NHE.
The excited-state lifetimes of K-73 and K-19 were determined
by transient absorbance measurements at 620 nm to be 24 ns
and 12 ns, respectively (Figure S6). As the molar extinction
coefficients of these two dyes are similar one expects the excited
lifetimes to be close in value. These differences might have
arisen due to the difference in the number of protons between
K-19 and K-73 dyes. As the electron injection from the exited
state dye is in the femtosecond range, the lifetime of the exited
state measured is sufficient to produce quantitative injection of
electrons into the TiO2 electrodes.
2.3. ATR-FTIR Measurements. The spectrum of K-73
anchored on the mesoscopic TiO2 film (Figure. S7) clearly
shows the bands at 1624 cm-1 and 1380 cm-1 for the
asymmetric and symmetric stretching modes of the carboxylate
group and the complete loss of the 1717 cm-1 peak. From these
ATR-FTIR data it can be inferred that the dye is anchored on
the surface through the two carboxylate groups via a bidentate
chelation or a bridging of surface titanium ions rather than an
ester type linkage.13 The NCS group absorption remains at 2101
cm-1 indicating that the NCS coordinated to the ruthenium
center through the N atom is unaffected by the adsorption
process. The other peaks at 2847 cm-1 and 2924 cm-1
correspond to the stretching modes of the methyl while an sp2
C-H stretching mode is observed at 3029 cm-1. The sharp
(14) Nazeeruddin, M. K.; Zakeeruddin, S. M.; Kalyanasundaram, K. J. Phys.
Chem. 1993, 97, 9607.
(15) Satzger, H.; Zinth, W. Chem. Phys. 2003, 295, 287.
(13) Shklover, V.; Ovehinnikov, Y. E.; Braginsky, L. S.; Zakeeruddin, S. M.;
Gra¨tzel, M. Chem. Mater. 1998, 10, 2533.
9
J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006 4149

A R T I C L E S
Kuang et al.
oxidized N3 dye corroborates that the transition responsible for
the broad absorption band centered around 750 nm is a ligand-
to-metal charge-transfer transition.
The rate of electron injection from K-19 into TiO2 is
illustrated in Figure 2A by the time vector associated to the
spectral vector discussed above. The oxidized state appears to
reach its maximum within the pulse duration. A small decay is
apparent. It is imputed to a degradation of the dye rather than
to recombination or any other chemical reaction since the
bleaching observed at 530 nm does not evolve in the same way.
This means that the ground state is not regenerated over the
time range of our experiment (0-2000 ps). The degradation is
likely to be due to the high pump intensities needed to get a
good spectral resolution. When the intensity is reduced (<0.5
µJ/pulse) the signal is perfectly stable, no evolution of the
transient species is observed (see inset of Figure 2A). Cleavage
of the C-S bond in the thiocyanate ligand as observed for N3
might be responsible for this behavior.16a Data are fitted with
an analytical convolution of a Gaussian function representing
the instrument response with an exponential decay. The fitted
full width at half-maximum (fwhm) of 70 fs allows us to give
an upper limit for the electron-transfer time constant of 20 fs
similar to the estimated value for deprotonated N3 in the absence
of slow components related to dye aggregation on the semi-
conductor’s surface.17
From the femtosecond time-resolved and spectrally resolved
data we can conclude that electron injection from K-19 to TiO2’s
conduction band is ultrafast and therefore not expected to be
hindered by any competitive process. In addition we observe
that the spectral signature of the oxidized state of K-19 is
identical to those of other N3 derivatives.
Very recently, Haque et al. reported that electron injection
could be significantly slower in complete solar cells occurring
with a half-life time of 150 ps.18 These authors suggested that
the retardation was a consequence of the influence of the
electrolyte upon the conduction band energetics of TiO2. To
examine this possibility we performed two-color pump-probe
transient absorption measurements of samples containing the
nonvolatile electrolyte employed in highly stable solar cells for
outdoor applications.8b After light excitation at 535 nm, transient
absorption was monitored at 890 nm where the contribution of
the oxidized dye largely dominates the signal. As depicted in
Figure 2B, no additional rise is observed after the pump pulse,
indicating that injection is completed within the pulse duration,
similar to the results obtained in the inert ionic liquid where no
evolution is observed during the first nanoseconds. However,
with the redox electrolyte, a biphasic decay is observed. The
slower part in the nanosecond time scale is attributed to the
diffusive regeneration of the oxidized dye by iodide. The faster
component of the reduction reaction (τ ) 1.9 ps (12%)) is
attributed to a static interaction between iodide and the
ruthenium dye, the time constant associated with this process
precluding diffusion of iodide.19
Figure 2. (A) Transient absorption on TiO2 at 530 nm following excitation
at 520 nm (O) and first base vector of the singular value decomposition
(0). The inset shows the time vector from an SVD realized on the data
obtained with reduced laser pump fluence (4). (B) Transient absorbance
of K-19 adsorbed on a TiO2 film in the presence of the nonvolatile
electrolyte. The sample is probed at 890 nm upon excitation at 535 nm.
The early-time dynamics are detailed in the inset. Data are fitted with the
analytical convolution function of a Gaussian instrument response with two
exponential decays, τ1 ) 1.9 ps (12%) and τ2 ) 12 ns (88%).
is almost exactly the same as the one obtained from detailed
analysis of the analogue dye N3.16 The time vector associated
to this component is given in Figure 2A.
The spectral shape and the time evolution of the unique
transient species present after excitation of K-19 on TiO2
indicates that it can be reasonably identified as the oxidized
state of the dye. The same experiment was performed on Al2O3
in order to obtain information about the excited state. Since the
conduction band edge potential of this material lies much higher
than the energy levels populated in the excited state of the dye
no electron injection is expected. Again an SVD was carried
out, and a single transient species was identified. The spectral
profile of the excited state is significantly different from the
one observed on TiO2 (Figure 1). Comparison with the
electrochemically oxidized K-19 absorption spectra (Figure S8)
allows us to assign the latter confidently to the photo-oxidation
product of K-19. The fact that the transient spectrum of the
oxidized state produced on TiO2 is very similar to that of the
5. Dye Regeneration. Laser transient absorbance measure-
ments were performed to understand the photon-to-current
conversion efficiency of DSCs that is strongly dependent upon
the kinetic competition between back electron transfer of injected
(16) (a) Moser, J. E.; Noukakis, D.; Bach, U.; Tachibana, Y.; Klug, D. R.;
Durrant, J. R.; Humphry-Baker, R.; Gra¨tzel, M. J. Phys. Chem. B 1998,
102, 3649. (b) Das, S.; Kamat, P. V. J. Phys. Chem. B 1998, 102, 8954.
(17) Wenger, B.; Gra¨tzel, M.; Moser, J.-E. J. Am. Chem. Soc. 2005, 127, 12150.
(18) Haque, S. A.; Palomares, E.; Cho, B. M.; Green, A. N. M.; Hirata, N.;
Klug, D. R.; Durrant, J. R. J. Am. Chem. Soc. 2005, 127, 3456-3462.
9
4150 J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006

High Molar Extinction Coefficient Heteroleptic Ru Complexes
A R T I C L E S
that of Z-907 in organic solvent electrolyte.20 Remarkably, the
regeneration reaction kinetics can be described by a single-
exponential decay with a first-order rate constant kr ) 1.1 ×
105 s-1. Although data for the charge recombination kinetics
could not be fitted accurately with a single-exponential decay,
a rate constant of kb ) 1.4 × 103 s-1 was approximated (Figure
3, trace a), leading to a dye regeneration yield of 99%.
6. Ionic Liquid Electrolytes, Effect of the Scattering Layer,
and Film Thickness on Photovoltaic Performance. Strikingly
high conversion efficiencies are obtained with the new sensitiz-
ers in full sunlight using ionic liquid (IL) electrolytes. Due to
their high stability and negligible vapor pressure, these solvent-
free redox systems play a key role for practical applications of
the DSC. Advantage is taken of the enhanced optical cross
section of the sensitizer to reduce the mesocopic TiO2 film
thickness alleviating the mass transfer limitations of the pho-
tocurrent, which are notorious for ILs due to their high viscosity.
Thus a K-19 stained TiO2 layer of only 6.8 µm thickness used
in combination with the binary ionic liquid electrolyte gave Jsc
) 12.15 mA cm-2, Voc ) 714 mV, FF ) 0.737, and η ) 6.4%.
This is an impressive performance for a solvent-free, transparent
DSC. The conversion efficiency increased from 6.4 to 7.1% by
adding a scattering layer of 4 µm thickness to augment the light
harvesting in the red and near-IR region increasing the photo-
current.
Figure 3. Transient absorbance decay kinetics of the oxidized state of K-19
dye, adsorbed on a mesoporous TiO2 film in pure EMITFSI inert ionic
liquid (a) and in the presence of present binary ionic liquid (PMII and
EMINCS) electrolyte (b). The dye was coadsorbed from a solution in
acetonitrile and tert-butyl alcohol (volume ration: 1:1) containing PPA (1:
1, molar ratio). Absorbance changes were measured at a probe wavelength
of 680 nm, employing 600 nm laser excitation (5 ns fwhm pulse duration,
25 µJ cm-2 pulse fluence). Continuous lines drawn on top of experimental
data are monoexponential fit curves with first-order rate constants k ) 1.4
× 103 s-1 (a) and k ) 1.1 × 105 s-1 (b), respectively.
electrons from the TiO2 conduction band to the oxidized dye
cations (S+) and the interception of S+ by the redox mediator.
This efficiency is in fact directly proportional to the dye
regeneration yield Φr, defined as
Further insight in the behavior of the solvent-free DSC based
on the new K-19 sensitizer was obtained by investigating the
effects of film thickness and light intensity. Figure S9 shows
the photocurrent response to a light on-off sequence produced
by opening and closing a mechanical shutter that blocks the
light beam. Under low light levels (<100 W/m2), the current
remains constant on a time scale of seconds, while, for the
thicker films, decay is noted when the intensity is raised to g300
W/m2 indicating a mass transport limitation of the photocurrent
under these conditions. Thinner films made of a 3.4 or 6.8 µm
thick colloidal layer and a superimposed 4 µm thick scattering
layer gave the best performance with the binary ionic liquid
electrolyte.
Φr ) kr/(kr + kb)
(2)
In eq 2, kr is the first-order rate constant of the dye regeneration
(eq 5) occurring in the presence of a redox couple (D+/D), and
kb, the rate constant of the recombination reaction (eq 4) that
takes place between the dye oxidized state S+ and the conduction
-
band electron ecb injected during the primary ultrafast photo-
induced charge separation process (eq 3).
S|TiO2 + hν f S*|TiO2, S*|TiO2 f
S+|TiO2 + e-cb(TiO2) (3)
Figure 4 and Table 1 show the effect of varying the colloidal
TiO2 particle layer thickness on the photovoltaic parameter (Jsc,
Voc, FF, η) keeping the 4 µm reflecting particle layer constant
and using K-19 sensitizer coadsorbed with PPA. The Voc
decreases with increasing film thickness, due to the augmenta-
tion of the surface area providing additional charge-recombina-
tion sites and enhancing the dark current. Moreover, for thicker
films the outer TiO2 particle layers do not contribute signifi-
cantly to the photogeneration of conduction band electrons due
to the filtering of light by the dyed particles located close to
the FTO glass. The sharing of photoinjected conduction band
electrons by these particles lowers their quasi-Fermi level and
hence the Voc. By contrast, both Jsc and η peaked at 6.8 µm,
while the fill factor remained constant.
S+|TiO2 + ecb- (TiO2) f S|TiO2
S+|TiO2 + D f S|TiO2 + D+
(4)
(5)
Hence, it is essential to scrutinize the dynamics of these
charge-transfer processes with time-resolved nanosecond laser
experiments.
Figure 3a shows the time evolution of the K-19 transient
absorbance anchored on a mesoscopic transparent TiO2 film in
the presence of EMITFSI redox-inactive ionic liquid. In the
absence of an electron donor, the decay of the transient
absorption signal recorded at 680 nm following pulsed laser
excitation reflects the dynamics of recombination of injected
electrons with the oxidized dye (S+). The kinetics of S+ transient
absorbance decay exhibited a typical half-life time of t1/2 ) 800
µs (Figure 3, trace a). In the presence of the binary ionic liquid
(PMII and EMINCS) electrolyte, the decay of the oxidized dye
signal was significantly accelerated with t1/2 ) 10 µs (Figure
3, trace b). The dye regeneration appeared to occur faster than
Electron transport and recombination in TiO2 particles and
at the TiO2/redox electrolyte interface have been investigated
by using various experimental techniques such as IMPS, IMVS,
(19) Marton, C.; Clark, C. C.; Srinivasan, R.; Freundlich, R. E.; Narducci
Sarjeant, A. A.; Meyer, G. J. Inorg. Chem. 2006, 45, 362-369.
(20) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi,
T.; Gra¨tzel, M. Nat. Mater. 2003, 2, 402.
9
J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006 4151

A R T I C L E S
Kuang et al.
proposed and applied to analyze the electron transport in TiO2
film and recombination at the TiO2/electrolyte interface in
DSC.26-29 The EIS investigation of the DSC device provides
valuable information for the understanding of photovoltaic
parameters (Jsc, Voc, FF, and η). Impedance spectra of DSC
devices with different film thicknesses (1 + 4, 6.8 + 4, and
13.6 + 4 µm) used in conjunction with the K-19 dye and the
above-mentioned binary ionic liquid electrolyte were measured
under one sun illumination at an open circuit potential or in the
dark at -0.68 V, shown in Figure 5. A typical EIS spectrum
exhibits three semicircles in the Nyquist plot or three charac-
teristic frequency peaks in a Bode phase angle presentation as
indicated in Figure 5. In the order of increasing frequency the
features are attributed to the Nernst diffusion in the electrolyte,
electron transfer at the TiO2/electrolyte interface, and redox
charge transfer at the counter electrode, respectively. The
features for electron transport in the TiO2 film and interfacial
recombination overlap in the frequency range from 102 to 103
Hz.
Figure 4. The detailed photovoltaic parameter (Jsc, Voc, FF, and η)
variations with different film thicknesses of transparent layer and keeping
constant 4 µm reflecting layer based on K-19 dye, and PPA as coadsorbent
and ionic liquid electrolyte.
The chemical capacitance (Cµ) and charge-transfer resistance
(Rct) were obtained by fitting the middle frequency semicircle
in the Nyquist plots. The Cµ, which is scaled to the film
thickness, and Rct, which is scaled to the inverse of the film
thickness, is shown in Figure S10. Under light illumination, we
see that the Rct has no obvious change with the inverse of the
film thickness. Since Cµ has a near linear relationship with film
thickness, this resulted in the shift of a characteristic peak to
lower frequency with an increase in the film thickness. In the
dark, the capacitance increases linearly with film thickness (d)
and the Rct decreases linearly to 1/d, resulting in an almost
constant response time. In both cases, when measured under
light at an open circuit potential (Figure 5a) or in the dark at
-0.68 V (Figure 5c), the diffusion of I3- ion in the electrolyte
slows down with an increase in film thickness shifting the lowest
frequency peak in the Bode plot to smaller values. This trend
also can be seen by the Nyquist plots (Figure 5b and 5d). The
electron diffusion time also increases with film thickness as
shown by the shift of the main peak in the 102 to 103 Hz range
of the Bode plot to lower frequencies. The Warburg diffusion
resistance for electron motion becomes clearly visible for the
thickest film at the high-frequency end of the corresponding
hemicircle in the Nyquist presentation.
Table 1. Detailed Photovoltaic Parameters (Jsc, Voc, FF, and η)
for Different Film Thicknesses Based on K-19 Dye and PPA as
Coadsorbent with a Solvent-Free Ionic Liquid Electrolyte
film
thickness
m)
Jsc
(mA/cm2)
Voc
efficiency
(%)
(
µ
FF
(mV)
1 + 4
9.75
11.26
12.65
13.2
11.8
10.68
0.752
0.748
0.745
0.746
0.768
0.768
747.87
740.98
734.67
718.08
707.4
5.49
6.25
6.92
7.07
6.41
5.7
2 + 4
3.4 + 4
6.8 + 4
10.2 + 4
13.6 + 4
692.89
photocurrent transient, and EIS.2,21-29 Changes in the TiO2 film
thickness could affect these important properties.30 Electro-
chemical impedance studies were performed to further elucidate
these factors for the present ionic liquid electrolyte system.
7. Electrochemical Impedance Spectroscopy Studies. Elec-
trochemical impedance spectroscopy (EIS) is a powerful tool
for identifying electronic and ionic transport processes in DSC.
Adequate physical models and equivalent circuits have been
(21) (a) Dloczik, L.; Ileperuma, O.; Lauermann, I.; Peter, L. M.; Ponomarev,
E. A.; Redmond, G.; Shaw, N. J.; Uhlendorf, I. J. Phys. Chem. B 1997,
101, 10281. (b) Schlichtho¨rl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J.
Phys. Chem. B 1997, 101, 8141. (c) Oekermann, T.; Yoshida, T.; Minoura,
H.; Wijayantha, K. G. U.; Peter, L. M. J. Phys. Chem. B 2004, 108, 8364.
(22) Kern, R.; Sastrawan, R.; Ferber, J.; Stangl, R.; Luther, J. Electrochim. Acta
2002, 47, 4213.
(23) (a) Frank, A. J.; Kopidakis, N.; van de Lagematt, J. Coord. Chem. ReV.
2004, 248, 1165. (b) Nakade, S.; Kanzaki, T.; Kubo, W.; Kitamura, T.;
Wada, Y.; Yanagida, S. J. Phys. Chem. B 2005, 109, 3480.
(24) Solbrand, A.; Lindstro¨m, H.; Rensmo, H.; Hagfeldt, A.; Lindquist, S. E. J.
Phys. Chem. B 1997, 101, 2514.
(25) (a) Schwarzburg, K.; Willig, F. J. Phys. Chem. B 2003, 107, 3552. (b)
Zaban, A.; Meier, A.; Gregg, B. A. J. Phys. Chem. B 1997, 101, 7985.
(26) Bisquert, J. J. Phys. Chem. B 2002, 106, 325.
(27) (a) Bisquert, J. Phys. Chem. Chem. Phys. 2003, 5, 5360. (b) van De
Lagematt, J.; Park, N. G.; Frank, A. J. J. Phys. Chem. B 2000, 104, 2044.
(c) Bisquert, J.; Zaban, A.; Greenshtein, M.; Mora-Sero, I. J. Am. Chem.
Soc. 2004, 126, 13550.
This observation is rationalized in terms of an increase in
the number of traps encountered by the conduction band
electrons with increasing film thickness. This slows down the
electron motion lowering the electron collection efficiency. From
the I-V data in Table 1, the current density augments only 30%,
i.e., from 9.8 mA cm-2 to 13 mA cm-2 upon increasing the
thickness of the transparent layer 6.8 times, i.e., from 1 µm to
6.8 µm. Thereafter, the current density actually decreases with
film thickness up to 13.6 µm despite enhanced light absorption,
which can be explained by slow electron diffusion and
consequently low electron collection efficiency.
8. Photovoltaic Performance. The photocurrent action
spectrum of the device coadsorbed with K-73 and DPA in
combination with a nonvolatile electrolyte is shown in Figure
6. The incident photon to current conversion efficiency (IPCE)
exceeds 70% in a spectral range from 470 to 620 nm, reaching
its maximum of 85% at 540 nm. Considering the light absorption
and scattering loss by the conducting glass, the maximum
(28) Fabregat-Santiago, F.; Bisquert, J.; Garcia-Belmonte, G.; Boschloo, G.;
Hagfeldt, A. Solar Energy Materials & Solar Cells 2005, 87, 117.
(29) Wang, Q.; Moser, J. E.; Gra¨tzel, M. J. Phys. Chem. B 2005, 109, 14945.
(30) (a) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller,
E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115,
6382. (b) Kang, M. G.; Ryu, K. S.; Chang, S. H.; Park, N. G.; Hong, J. S.;
Kim, K. J. Bull. Korean Chem. Soc. 2004, 25, 742. (c) Park, N. G.; van de
Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2000, 104, 8989. (d) Ito, S.;
Kitamura, T.; Wada, Y.; Yanagida, S. Solar Energy Materials & Solar
Cells 2003, 76, 3.
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4152 J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006

High Molar Extinction Coefficient Heteroleptic Ru Complexes
A R T I C L E S
Figure 5. Impedance spectra of DSC devices based on different film thickness measured under 1 sun illumination at open circuit potential (a, b) or in dark
at a forward bias of -0.68 V (c, d). Bode phase plots: (a) and (c); Nyquist plots: (b) and (d). Black: (1 + 4) µm film. Red: (6.8 + 4) µm film. Green:
(13.6 + 4) µm film. The inset of Figure 5b is the equivalent circuits of DSC.29 (A) electron transfer at the FTO/TiO2 interface; (B) electron transport and
-
back reaction at the mesoscopic TiO2/electrolyte interface; (C) diffusion of I3 in the electrolyte; (D) charge transfer at the electrolyte/Pt-FTO interface.
Table 2. Detailed Device Parameters Based on (6.8 + 4) µm TiO2
Double Layer Film Sensitized with K-19/PPA and Using Ionic
Liquid Electrolyte under Various Incident Light Intensitiesa
Pin
(mW/cm2)
Jsc
(mA/cm2)
Voc
Pmax
(mW/cm2)
η
(%)
(mV)
FF
99.4
51.7
30
13.2
7.4
4.3
718
703
688
7.0
4.0
2.4
0.746
0.789
0.802
7.1
7.8
8.0
a Pin: Incident power intensity. Jsc: Short-circuit photocurrent density.
Voc: Open-circuit photovoltage. Pmax: Maximum electrical output power
density. FF: Fill factor (FF ) Pmax/Pin). η: Total power conversion
efficiency. Cell active area tested with mask: 0.158 cm-2. The spectral
distribution of the lamp (incident light source) simulates air mass 1.5 solar
light.
The photovoltaic parameters (Voc, Jsc, and FF) of the device
prepared with a 6.8 + 4 µm film and K-19 dye coadsorbed
with PPA in combination with a solvent-free ionic liquid
electrolyte are 718 mV, 13.2 mA cm-2, 0.745, respectively,
yielding an overall efficiency (η) of 7.1%. It is worth noting
that the photoelectric conversion efficiency reached as high as
8.0% under an irradiance of 30 mW cm-2. The detailed
parameters (Jsc, Voc, FF, and η) of the device under different
light intensities are shown in Table 2.
Figure 6. Photocurrent density-voltage curves of the DSC device based
(6.8 + 4) µm film sensitized with K-73/DPA and nonvolatile electrolyte
under AM 1.5 full sunlight (100 mW cm-2). The inset is the photocurrent
action spectra of the same DSC device. Cell active area: 0.158 cm2.
efficiency for absorbed photon to current conversion efficiency
is practically unity over this spectral range. As shown in Figure
6, the short-circuit photocurrent density (Jsc), open-circuit
photovoltage (Voc), and fill factor (FF) of the K-73 dye based
device under AM 1.5 full sunlight are 17.22 mA cm-2, 748
mV, and 0.694, respectively, yielding an overall conversion
efficiency (η) of 9%. At lower light intensities, the overall power
conversion efficiencies are over 9.5%. These are the highest
conversion efficiencies ever reported for DSCs based on
nonvolatile electrolytes. It is important to note that this
impressive performance is achieved with thin films, the nanoc-
rystalline TiO2 layer being only 6.8 microns thick, vindicating
our approach for developing heteroleptic high extinction coef-
ficient sensitizers.
The aim of designing high molar extinction coefficient
sensitizers is to obtain high efficiencies with thinner mesoscopic
TiO2 films. To demonstrate that this strategy works, we chose
for comparison the Z-907Na dye which has a 33% lower molar
extinction coefficient than that of K-19 dye, by using a 3.4 µm
transparent TiO2 film in combination with a binary ionic liquid
electrolyte. Using the K-19 dye we obtained a 19% higher
current density (10.9 mA cm-2) than measured with Z-907Na
(8.8 mA cm-2) at full sun irradiance resulting in an overall
efficiency increase of 20% (from 5% to 6%). This result
9
J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006 4153

A R T I C L E S
Kuang et al.
Figure 8. Photovoltaic parameter (Jsc, Voc, FF, and η) variations with aging
time for the device based on (6.8 + 4) µm film sensitized with K-19/PPA
and ionic liquid electrolyte during successive one sun visible-light soaking
at 60 °C.
Figure 7. Photovoltaic parameter (Jsc, Voc, FF, and η) variations with aging
time for the device based on (6.8 + 4) µm film sensitized with K-73/DPA
and nonvolatile electrolyte during successive one sun visible-light soaking
at 60 °C.
Conclusions
undoubtedly demonstrates the advantage of high extinction
coefficient sensitizers in DSC.
The present study confirms the advantage of high extinction
coefficient sensitizers in realizing efficient thin film solar cells.
The two novel high molar extinction coefficient ruthenium dyes
with (K-19) and without amphiphilic chains (K-73) presented
show very attractive features with regards to photovoltaic
performance and stability. Devices based on the K-73 dye and
a nonvolatile electrolyte yielded an unprecedented overall
conversion efficiency of 9% under AM 1.5 global sunlight. Solar
cells employing the K-19 dye in combination with a binary ionic
liquid electrolyte gave over 7.0% efficiency and maintained
excellent stability under light soaking at 60 °C for 1000 h. Work
is in progress to enhance further the photovoltaic performance
and long-term stability of DSCs based on nonvolatile or solvent-
free ionic liquid electrolytes for which practical applications
can be foreseen soon.
9. Device Stability. The K-73 dye showed good stability
when subjected during 1000 h to light soaking at 60 °C in the
solar simulator (Figure 7). However during long term heating
at 80 °C a desorption of the sensitizer from the surface was
noted. In contrast, the K-19 dye showed excellent stability under
both light soaking at 60 °C and thermal stress at 80 °C.8 Thus
it appears that the adsorption of the sensitizer is strengthened
by the presence of the long alkyl chains on the K-19 dye,
producing high thermal stability in the presence of the non-
volatile electrolyte employed here.
Devices employing K-19 in conjunction with ionic liquid
electrolytes showed also excellent long-term stability, when
subjected to an accelerated light soaking test at 60 °C. Figure
8 presents the stability data of the device containing K-19 dye
and PPA as coadsorbent which kept 93% of the initial
performance after the 1000 h light soaking test. It is worth noting
that after a week of light soaking at 60 °C the efficiencies of
DSC at full sun and 0.3 sun illumination increased to 7.3% and
8.3%, respectively. After 1000 h of light soaking, the Voc
decreased by 60 mV but this loss is compensated by a gain in
short circuit current density from 13.2 to 14.5 mA cm-2. The
long-term stability of the device at low light intensity (0.3
sunlight) is even better than 1 sunlight intensity; the initial
efficiency of 8.0% is decreased to 7.8% during the 1000 h light
soaking test. The use of the novel high extinction coefficient
sensitizers in conjunction with ionic liquid appears to also
provide stability advantages at low light intensities.
Acknowledgment. We are grateful to Dr. Qing Wang and
Dr. Augustin McEvoy for helpful discussions, P. Comte and
R. Charvet for the mesoscopic TiO2 paste fabrication, T.
Koyanagi (CCIC, Japan) for providing the 400 nm sized TiO2
particles, and The Swiss Science Foundation and Swiss Federal
Office for Energy (OFEN) which have supported this work.
Supporting Information Available: Detailed characterizations
of K73 dye, device photocurrent transient, film capacitance, and
charge-transfer resistance for different film thickness. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA058540P
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4154 J. AM. CHEM. SOC. VOL. 128, NO. 12, 2006